On 12/14/2013 12:46 PM, Tony Hwang wrote:
Nate Nagel wrote:
On 12/14/2013 11:21 AM, Nate Nagel wrote:
On 12/14/2013 10:52 AM, Ian wrote:
I am installing a new NEST thermostat, replacing an old two wire
mercury bulb furnace thermostat. There are only two unmarked wires.
There's 24 V across the wires.

Can anyone tell me which letters on the NEST base (W, Y, O/B, AUX, E,
G, C, etc) they will map to? It has to be the most basic of
installations; open or close the circuit.

Thanks,

Ian


http://www.behvac.com/troubleshooter41.htm

You'll probably need to repull the thermostat wire with at least three
wires so you have a common available to power the thermostat; I would
repull with 5 conductor in case you think you may want to add central
air in the future.

In short, one of the existing wires connects to "R" (if you have "RH"
and "RC" then connect to "RH") use "RED" wire for this connection. This
one will be the one coming directly from the transformer.

The other existing connection connects to "W" - use the "WHITE" wire for
this one.

Finally you will need another conductor from the 24VAC side of the
transformer to the new thermostat, that will connect to "C" for Common
and if there is a black wire in your cable use that one.

It says here that you "may" not need to repull if you don't have a "C"
wire

https://nest.com/thermostat/installation/

Looking at the picture there, it appears the terminals you would use are
RH, W1, and C.

here's more info on why you may want to repull

http://support.nest.com/certified/ar...-set-up#common




Hope this helps

nate



Forgot to add; if you repull then you can also use the fan-only mode.
Use the green wire for that function.

nate

Hi,
Why the trouble repulling wires? Some times it is almost impossible
pulling extra wires. In that case go wireless.

In my last house it took longer to puzzle out the proper connections
within the furnace than it did to pull the thermostat wire. Open
ceiling in basement, and thermostat wire was clearly visible going
through floor, only had to do about 4 feet "blind." If that's the OP's
situation there's really no reason not to do it.

Obviously if it's a complete PITA to do so another solution might be
preferable.

nate

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